In electronics, an opto-coupler, also referred to as an opto-isolator, photocoupler, or optical isolator, is an optoelectronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output. One goal of an opto-coupler is to prevent high voltages or rapidly changing voltages on one side of the circuit from damaging components or distorting transmissions on the other side.
A typical opto-coupler includes a light source, such as a Light Emitting Diode (LED), a photodetector, and an insulation medium. As the name suggests, an optical path needs to be created between the LED and photodetector via the insulation medium. This is traditionally done by using an optically-transparent material such as silicone to create the light path. The insulation medium not only acts to allow the transmission of light from the LED to the photodetector, but the insulation medium also electrically insulates the input and output sides of the circuit.
Certain applications have stringent design rules regarding the true distance between the high voltage and low voltage side of the circuitry. In opto-couplers, the true distance between the high voltage side and low voltage side of the Printed Circuit Board (PCB) translates to be the closest metal-to-metal distance within the opto-coupler. This distance is often referred to as the opto-coupler's Distance Through Insulation (DTI), creepage distance, or the like. It should be appreciated that the DTI of opto-couplers is an important design consideration/constraint.